This invention relates to unstable optical resonators for laser applications and more particularly to reflective surfaces in an unstable resonator system.
An unstable optical resonator is one which has divergent focusing properties such that the optical modes fall in the unstable or high loss region of the resonator mode chart. Unstable optical resonators have been described in the following published articles: (1) "Unstable Optical Resonators for Laser Applications" by A. E. Siegman, Proceedings of the IEEE, March 1965, pp 277-287; (2) "Modes in Unstable Optical Resonators and Lens Waveguide" by Anthony E. Siegman and Raymond Arrathoon, IEEE Journal of Quantum Electronics, Vol. QE-3, #4, April 1967, pp 156-163; (3) "Properties of An Unstable Confocal Resonator CO.sub.2 Laser System", by William F. Krupke and Walter R. Sooy, IEEE Journal of Quantum Electronics, Vol. QE-5, No. 12, December 1969, pp 575-586.
Unstable resonators have a large diffraction loss but are useful because they have large mode volumes in very short resonators, the unstable configuration is readily adapted to adjustable-diffraction output coupling and they have substantial discrimination against higher-order transverse modes.